Six years ago, to pass a state budget, the Legislature”s Democratic leaders agreed to place a major change in primary election voting before voters.

They”ve regretted it ever since.

Proposition 14, approved by voters in 2010, is a “top-two” primary system in which all candidates appear on the same ballot in June and the two highest vote-getters, regardless of party, face each other in November.

Its practical effect has been that in some legislative districts, two Democrats wind up in runoffs, thus allowing Republican and independent voters to tip the balance in favor of the less liberal of the two.

Over the past two election cycles, the system — coupled with redistricting by an independent commission — helped create a bloc of moderate, business-friendly Democrats in the Assembly, and it”s beginning to have the same effect in the Senate.

The bloc has dealt big setbacks to liberal groups — and even Gov. Jerry Brown — in their legislative agendas.

So the top-two system is working as its supporters intended at the legislative level. But could it have the same effect in statewide elections?

Democrats dominate those elections, and that”s not about to change. But the top-two system could come into play under the right circumstances: two viable Democrats running against each other with weak Republican candidates.

And that, political scientist John Pitney says, could happen next year in California”s only statewide contest, filling the U.S. Senate seat being vacated by Barbara Boxer.

Attorney General Kamala Harris is the odds-on favorite to win the seat, but U.S. Rep. Loretta Sanchez, D-Garden Grove, is also running. With three underfinanced Republicans also seeking the seat, the situation is ripe for Harris and Sanchez to finish 1-2 in the June primary and face each other in November.

“There are several reasons Sanchez could win that showdown,” Pitney, a professor at Claremont McKenna College, asserts in a recently published article.

Sanchez is more of an ideological centrist, as shown by her most recent spate of endorsements, and thus would more naturally draw support from business and conservative groups, as well as Republican voters.

The gender divide would be neutralized, and Sanchez could pull Latino votes away from Harris.

As Pitney points out, were the Republicans to coalesce around one GOP candidate, the chances of a Harris-Sanchez runoff would diminish, but it”s at least possible, and perhaps even probable, that it will occur.

Were it to happen, it would demonstrate the potential of top-two to affect statewide races, setting the stage for 2018, when a multi-Democrat contest for governor is shaping up.

It”s questionable whether an even semi-strong Republican will be running for governor; thus, it”s possible that Republican voters, as diminished as their ranks have become, could be decisive not only in next year”s Senate race but in the contest for governor as well.